1. Field of the Invention
The present invention generally relates to the removal of coating material from various objects. More particularly, the invention concerns a method and apparatus to recycle documents by using a laser to erase toner, ink, or similar material from the documents.
2. Description of the Related Art
Paper recycling is an important part of the present recycling industry. Recycled paper is used in greeting cards, tissue paper, cardstock, and a variety of other paper-based products. With increasing worldwide concern for the environment, interest in paper recycling is higher than ever.
The primary system for recycling paper today involves a "re-pulping" approach, which uses a combination of chemical and mechanical means. In this system, a large volume of used paper is typically ground into a mash, treated with certain known bleaching chemicals, and then reconstituted into sheet form.
As an alternative to the "re-pulping" system, many have attempted to recycle paper using various non-destructive whitening methods. Broadly, known non-destructive whitening methods clean printed paper "as is", without breaking the paper down into pulp and pressing and drying it back into sheet form. Known whitening methods typically employ chemical agents to dissolve toner, bleaching agents to whiten paper, and chemical and/or mechanical pealing agents to remove printing.
For some recycling applications, the known whitening methods may be adequate. However, for many users these techniques are not completely satisfactory. These methods, for example, often use dangerous chemicals that may be poisonous to humans. This presents a significant risk of injury to recycling personnel, and also requires comprehensive chemical handling and disposal procedures. Moreover, some users may find non-destructive whitening methods to be too slow and expensive for their applications. Also, these techniques may damage the paper surface, rendering the paper unusable for some purposes.
Prior to the present invention, the use of lasers in document recycling has been unknown. In a completely dissimilar field, however, lasers have been used to perform "surface cleaning" of various hard, opaque objects. Several companies, for instance, use lasers to remove paint from metal surfaces of airplanes and ships. Others have applied lasers to clean the surface of stonework, buildings, and other hard opaque objects.
As shown in FIG. 1, these prior surface cleaning techniques operate simply by impinging a laser beam 100 onto the undesirable coating 102. The coating 102 is adhered to an underlying object 104. When the laser 100 contacts the coating 102, the laser is absorbed by the coating 102 which then creates a plasma shock wave 200 (FIG. 2). Although the plasma shock wave 200 is oriented toward the object 104, the wave 200 and the underlying object 104 cooperate to discharged a significant portion 202 of the coating proximate the laser beam 100. The discharged material 202 is effectively removed from the object.
For many applications, the known laser-based surface cleaning techniques satisfy their user's expectations. However, the present inventors have discovered that certain characteristics of these methods yield poor performance when applied to the task of paper cleaning. With a rigid surface such as the aluminum skin of an airplane (e.g. FIGS. 1-2), the underlying object 104 effectively repels the coating 202 proximate the laser beam 100. However, the portion of coating 204 directly impinged by the laser beam 100 is not repelled so effectively. In fact, some of the coating 204 may be impregnated into the object 104. In the case of a rigid surface such as a metal substance, this impregnation is minimized due to the difference in hardness between the relatively hard object and the relative soft coating.
The inventors have discovered that, unlike hard surfaces such as metal, paper is quite susceptible to coating impregnation during conventional laser-based surface cleaning. As a result, the impinging laser not only does a poor job of removing the coating, but it also causes a significant level of paper damage. As a result, an improved paper recycling technique is still needed.